EMRFD Message Archive 13241
Message Date From Subject 13241 2016-11-01 09:57:08 Mark Meyer Non-inductive resistors from China I recently bought some non-inductive resistors from China for use as a dummy load for QRP work. They are rated 50 ohms and 100W. They do measure 50 ohms.The resistors are composed of a ceramic chip on a metal base. The chip is about 10mm long and 5mm wide. The metal base with bolt holes is about 20mm long. A flimsy lead comes out of the side of the chip and the other connection is the metal base. The markings on the chip are100A1, 0803, and a company logo with HB inside a diamond.These seem very flimsy to me and I don't see how they could possibly be rated 100W. Has anyone had any experience with this type of device?Mark WU0L 13242 2016-11-01 10:11:34 Dana Myers Re: Non-inductive resistors from China 13243 2016-11-01 10:30:49 DuWayne Schmidlko... Re: Non-inductive resistors from China
It is all in the size of the heatsink you use with them, bigger is better. I put pictures of a 200 watt commercial dummy load using similar resistors in a dropbox folder. One shows the overall dummy load and the other the resistor mounting.
13244 2016-11-01 12:33:15 Graham / KE9H Re: Non-inductive resistors from China Same case style as a 200 Watt Transistor, which if running 50 percent efficient will dump 100W into the base plate.Mount it like you would a 200 Watt Transistor. All the same rules apply. This is a common case style for VHF/UHFdummy loads.--- Graham / KE9H== 13245 2016-11-01 13:30:08 kerrypwr Re: Non-inductive resistors from China As said, it's the heatsink that counts.
I used two 100R/30W Diconnex resistors of this kind in a small power meter;
The resistors are seen at lower centre, mounted back-to-back.
It was only intended for HF so the layout is nothing special but VSWR is good; 1:1 at HF, increasing to 2:1 at 700 MHz.
I also built a larger load/power meter using a Florida RF resistor; 250W I think;
I don't have photos of the interior but the resistor sits in a "pocket" milled in one heatsink; the two heatsinks are bolted back-to-back.
It handles 100W (my limit) easily; its limit for continuous power is probably 150-200W.
Although I used co-ax for connections, these resistors are intended for use with microstrip/stripline.; the body bolts to the heatsink whilst the tab connects to the stripline.
If using co-ax, ensure that the outer is mounted solidly as any movement can break the resistor tab.
Also, put a little "kink" in the tab; that allows it to expand & contract slightly as it heats or cools. If it's left straight, the forces can break it or its connections. I think that is covered in the Florida RF website which has installation notes.
13246 2016-11-01 15:23:54 Nick Kennedy Re: Non-inductive resistors from China I got some similar to your description from eBay at $5.35 for 5 ea, postpaid. I'm similarly incredulous at the 100 W rating for the size. They're supposed to be good up into the GHz region too. They're "pulls', I guess, so the little lead is even shorter but should be usable.I'd imagine it will take the mother of all heat sinks with mounting just perfect to handle that amount of power continuously.It got me to wondering how you would tell if you didn't have sufficient heat sinking capacity, short of smoke and a fireball. If resistance begins to take off, one might power it from DC for a test and monitor the current, stopping before destructive overheating occurs. Hmmm ...73-Nick, WA5BDU 13247 2016-11-01 16:03:30 kerrypwr Re: Non-inductive resistors from China Yes; pretty amazing examples of modern materials technology.
The DC test is quite valid; I've done it on my projects. A power supply of 70V/1.5A will test for 100W; the supply I have used is 65V which equates to 85 watts, fairly close for assessing a 100 watt load.
Very basic design helps; simple maths says that we want to dissipate X watts and figures of watts/degree of temperature rise are available for many heatsinks.
For example, the heatsinks I used in my "100 watt" load have thermal resistance figures of about 1C/watt and 0.8C/watt. Let's guess at 0.6C/watt for the combination.
So 100 watts will raise the temperature 60C; at, say, 25C room temperature that gives 85C.
My tests showed that that is about right, perhaps conservative as the temperature was a little less than 85C.
The centre of the heatsinks, where the resistor is, can be touched at 100 watts; the ends are a little cooler. The resistor will be much hotter than this but it's designed to take high temperature.
Now this kind of calculation is very crude and doesn't take account of other factors as a "proper" heatsink design would but it's a quick way of assessing heatsink requirements for less-critical uses.
13248 2016-11-01 16:28:34 Mark Meyer Re: Non-inductive resistors from China The Mouser ones look very similar. Perhaps with a good heat sink they are okay. I won't be running that much power to them unless I try building a 200W dummy load with a series parallel combination. I have 5 of them. I also have several good sized heat sinks laying around.Thanks Mark WUoL 13250 2016-11-01 18:42:13 Jim Strohm Re: Non-inductive resistors from China For the rest ... Vishay sources some non-inductive TO-220 case resistors and claim ratings well into UHF. They are inexpensive for new ISO-9000 warranteed parts. DigiKey or Mouser; overseas customers contact me in private email.Kerry,That's a really nice looking power meter.
I put one on a heatsink with an SO-239 and the shortest lead dress I could kludge up. It works FB for HF and QRP, but I have not yet done serious testing. My MFJ-269C likes it a LOT to the top of its UHF scale. Someday I'll sweep it and see for real.
13252 2016-11-01 19:05:44 kerrypwr Re: Non-inductive resistors from China It sounds as if your Vishay resistors are good but not all "non-inductive" resistors are so at RF since most of them are designed for power supply use where "high frequency" can mean 100 kHz.
Frequency response is not usually given in the data sheets for these resistors; it is given in some form, even as basic as "up to 1 GHz", for true RF resistors.
The other thing to watch is power dissipation; many such resistors cannot withstand much heat so, even on a heatsink, must be substantially de-rated, sometimes to 50% or worse. Always study the power rating curve that is included in most data sheets.
The "RF" ceramic resistors seem to be capable of withstanding very high temperatures.
13253 2016-11-01 19:17:11 kerrypwr Re: Non-inductive resistors from China For dissipation greater than about 200 watts, I think that free-air cooling becomes impracticable and forced air (fan) cooling is required.
I remembered that there was a good discussion on high-power loads here;
Bryan WA7PRC has shown how to build a QRO-power load;
13254 2016-11-01 23:40:04 John Re: Non-inductive resistors from China The 100W rating means it can handle the 2 amp current which would
produce 100W/50 ohms. It depends on the user installing it on a
sufficient heat sink to remove the heat.
100A1 is probably a part number indicating 100W, to the 100W claim on
ebay is probably the manufacturers claim, not just the ebay seller. the
0803 quite possibly the date code.
13255 2016-11-02 13:22:21 Sandeep Lohia Re: Non-inductive resistors from China bit off topic,which glue to mix with heatsink paste?wana solidify it around SMD / MFR.
Please encourage recycling, reuse or repairing of E-waste.
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13257 2016-11-03 06:00:19 AncelB Re: Non-inductive resistors from China Don't those units require a strong clamped & lapped contact with the
13259 2016-11-03 12:30:53 kerrypwr Re: Non-inductive resistors from China This is what Florida RF recommend;
13275 2016-11-07 06:37:17 wa5bdu Re: Non-inductive resistors from China Since I'd recently bought some of these loads from eBay